Ocular melanoma is the most common primary cancer of the eye. It occurs most often in lightly pigmented individuals with a median age of 55 years. However, it can occur in all races and at any age. One of the effective managements of ocular melanoma is an invasive localized radiation treatment, most commonly performed via radioactive plaque brachytherapy. The plaque is a small gold cover or shield device containing low energy radioactive seeds, and is inserted into a plastic carrier. The plaque is sutured to the wall of the eye (sclera) beneath the base of the tumor with the patient under local or general anesthesia. The typical irradiation time is three to four days. The plaque is removed surgically after the completion of treatment. Its effectiveness was demonstrated by the Collaborative Ocular Melanoma Study (COMS) in a clinical trial funded by the National Eye Institute of the National Institutes of Health. The study began in 1986 and was carried out in a multi-institution setting.
The success of brachytherapy inspired efforts of using focused external photon radiations, x-rays, or gamma rays for localized radiation delivery to ocular melanoma with less or non-invasiveness. The treatment delivery systems and methods resulting from these efforts include GAMMA KNIFE® (Elekta AB, Stockholm, Sweden), CYBERKNIFE® (Accuray Inc., Sunnyvale, Calif.), and other systems incorporating medical linear particle accelerators. The major challenge of such systems and techniques, however, is the control of the patient's unpredicted eye movement. Although stereotactic radiation treatment with external photon beams has been attempted, there is no generally available robust eye motion-tracking mechanism. Active control of eye movement requires some level of invasive procedure, such as supra- and infraorbital nerve blocking and tethering sutures to the periorbital tissue. While some investigators carried out their treatments without eye fixation or with peribulbar injection of lidocaine for eye movement control, the accuracy of treatment delivery is inevitably compromised due to un-accounted-for eye movement during treatment.
CYBERKNIFE is an image-guided robotic radiosurgery system developed in the early 1990s and has reached technical maturity in recent years. The system utilizes modern diagnostic image processing techniques that delineate treatment target volume and follows a focused radiation dose plan, subsequently used for guiding a light-weight medical linear accelerator to deliver radiation to the tumor. The linear accelerator of 6 MV x-rays is mounted on a high-performance robot. With an onboard image guiding system, consisting of kV x-ray stereotactic imager (XSI) and a motion monitoring system (SYNCHRONY®, Accuray Inc., Sunnyvale, Calif.), small beams of radiation can be directed to the treatment target from a wide range of angles with sub-millimeter accuracy. The focused radiation beams from multiple directions (typically 100 to 200 beams, each beam lasting several to tens of seconds) create a highly concentrated radiation dose distribution with the ablative dose covering the treatment target and with the surrounding normal tissues being minimally exposed. The SYNCHRONY system can generate a mathematical model that predicts the tumor motion by correlating it with an external optic motion monitoring system. The information of tumor motion is then fed through the integrated control system to guide the robot to follow the treatment target in real time. Currently, the CYBERKNIFE real-time motion tracking system can only manage a tumor's “regular” movement resulting from the periodic respiratory motion.
The CYBERKNIFE system may include a motion simulation phantom. The motion simulation phantom can generate program-controlled movements with various speeds and magnitudes, which may be tracked by the motion monitoring system SYNCHRONY. The tracking accuracy of CYBERKNIFE falls within 0.5 mm during a 10-minute test.
The treatment of intracranial targets with CYBERKNIFE involves a 6D skull tracking technique. Stereotactic x-ray images are acquired repeatedly prior to turning on each treatment beam, the location of the tumor is updated and the aiming of the radiation beam is then adjusted. This is adequately accurate given the fact that the patient's head is immobilized and the intracranial targets are generally stationary in relation to the skull. When treating an intra-ocular target, the situation becomes more complicated due to the motion of the eye. The only way to accurately, precisely, and safely delivery high doses of radiation to the intraocular tumors without invasive immobilization of the eye would be to redirect the radiation beams to the tumor's updated position with sufficient frequency. Given the CYBERKNIFE's flexible mobility and the system's real-time motion response and robust control, minute and swift motion compensation by the CYBERKNIFE robot during the treatment of ocular tumors is practically achievable.
It is desirable to provide a non-invasive approach that can provide accurately localized radiation treatment for ocular melanoma or other intraocular indications through the completion of specified research tasks. It is further desirable to provide a surrogate and its associated data transformation system such that the changed tumor position can be dynamically localized.